Synthetic Theory of Evolution

The synthetic theory of evolution describes the evolution in terms of genetic variations in a population that leads to the formation of a new species. It explains the contribution of factors such as genetic variations, reproductive and geographical isolation, and natural selection. It is a modern concept that emerged from subsequent development of Neo-Darwinism from Darwinism.

Development of Synthetic Theory of Evolution

Darwinian evolution is comparatively an older concept. It developed the idea of evolution and origin of species based on the theory of natural selection i.e survival of the fittest and removal of the unfit ones with the course of time. It talked about variation, albeit didn’t give any idea on the causes of variability.

Darwinism was later merged with data obtained through genetic research. As a result, Neo-Darwinism has developed. And through further synthesis of Neo-Darwinism, the modern synthetic theory of evolution has emerged. It explained the reasons behind variations in offsprings.

Good to know

Blending inheritance is an obsolete theory in biology from the 19th century. The theory is that the progeny inherits any characteristic as the average of the parents’ values of that characteristic. Such as: son will get the average height of his parents’ heights.

There were 3 groups of supporters of Darwinian evolution.

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  • Field naturalist.
  • Paleoontologist (These two groups concentrated only on observation).
  • Geneticists: Unlike the other two, they were dependent on experiment and these people didn’t agree with natural selection as the cause of evolution. The field naturalists thought that graphical isolation was the cause of evolution.
    The geneticists proved that “Blending theory of Darwin was wrong – Meanwhile De Vries discovered mutation in the plant Oenothera lamarckiana. They believed that mutation was very necessary for evolution. Mutation brought a big and discontinuous variation. This idea was just opposite to Darwinism.

Then the Synthetic theory of evolution appeared where all the questions of scientific criticism of Darwinism were answered.

6 scientists are considered the founder of Synthetic theory:

  1. Theodosius Dobzhansky (1937)
  2. Julian Huxley (1942)
  3. Ernst Mayer (1942)
  4. G.G. Simpson (1944)
  5. Bernard Rensch (1947)
  6. G. L. Stebbins (1950).

Synthetic theory according to Stebbins (1950)

G. Ledyard Stebbins.

George Ledyard Stebbins Jr. (January 6, 1906 – January 19, 2000) was an American botanist and geneticist who is widely regarded as one of the leading evolutionary biologists of the 20th century. Along with Dobzhansky (1900 – 1975), animal systematist Ernst Mayr, and paleontologist George Gaylord Simpson (1902 – 1984), Stebbins is considered one of the “architects” of the modern evolutionary synthesis of the 1930s and 1940s, an intellectual watershed and historic turning point that brought together research in cytology, genetics, systematics, paleontology into a common evolutionary framework.

He compared the synthetic theory with a motor car. According to him,

Main factors of evolution

Gene mutation – Petrol

The changes that occur in the gene due to phenotypic effect is known as mutation. This produces a variety of changes that may be harmful. It causes variations in the offspring.

Chromosome mutation – Transmission or, acceleration

Chromosomal mutations like deletion, inversion, duplication, translocation, polyploidy result in the recombination.

Genetic recombination – Engine

Mainly occurs as crossing over during meiotic pachytene phase between homologous chromosomal pairs. It produces a new mix up of genes that creates offspring with a new combination of characters.

Natural selection – Driver

Natural selection produces a change in the frequency of the genes from one generation to the other favoring the differential form of the reproduction. The natural selection process creates an adaptive relation between the environment and the population through various combinations of genes. It does not produce genetic change, but once it has occurred it acts to encourage some genes over others.

Sexual isolation – Speed limit or direction of road.

It is one of the significant factors responsible for the synthetic theory of evolution. Isolation may be caused by physical barriers like rivers, oceans, high mountains, sometimes even the differential water current within a river system. The isolation helps in preventing the interbreeding of related organisms.

Accessory power or accessory factors of evolution

These factors cause the evolution to happen faster.

  • Migration
  • Introgressive hybridization
  • Genetic drift

According to him, evolution process on the basis of Natural selection occurs in two steps

Origin of heritable variants

An isolated population of a species independently develops different types of mutations (discussed above). The latter accumulate in its gene pool and creates variants within species.

Selection among variants

It is actually triggered by natural selection. The variants within a species are not equally adapted to the change of environments. So, naturally, the environment acts as a filter that results in the survival of the fittest ones and removal of the unfit members.

After several generations, through selection and mutations, the variants increases many folds and cause to originate a different species that is significantly different from the original one.

At a glance

  • Synthetic theory of evolution = Darwinian evolution + Genetics.
  • Good to know: Stebbins entered Harvard University in 1924 intending to study law, but his interests quickly changed to botany. He graduated from Harvard with a Ph.D. in 1931.


  • https://ucjeps.berkeley.edu/bryolab/babs/Stebbinsobit.html
  • https://byjus.com/biology/modern-synthetic-theory-evolution/
  • Featured Image: https://qz.com/1600349/how-genetic-tests-like-23andme-fuel-white-supremacy/
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